I would like peoples opinions on how to deal with the following scenarios:

The first, a guy grabs you from behind round the waist and you have at least one arm free

The second, you are grabbed from behind in the same manner but both arms are trapped

The third as you are grabbed his forearm is across your throat and crushing it

The fourth as above but he also has a knife at your throat

In all cases you are unarmed, there is no accomplice to the attacker and there is no one else around to help you.